如果我从 MySQL 父表中删除一行会发生什么?
mysqlmysqli database
从父表中删除行时，如果子表中使用了该行的数据，则 MySQL 将因 FOREIGN KEY 约束失败而抛出错误。可以通过名为"customer"和"orders"的两个表的示例来理解。这里，"customer"是父表，"orders"是子表。我们无法从"customer"表中删除子表"orders"中使用的行。可以通过从父表中删除值来演示，如下所示 −
mysql> Select * from Customer; +----+--------+ | id | name | +----+--------+ | 1 | Gaurav | | 2 | Raman | | 3 | Harshit| | 4 | Aarav | +----+--------+ 4 rows in set (0.00 sec) mysql> Select * from orders; +----------+----------+------+ | order_id | product | id | +----------+----------+------+ | 100 | Notebook | 1 | | 110 | Pen | 1 | | 120 | Book | 2 | | 130 | Charts | 2 | +----------+----------+------+ 4 rows in set (0.00 sec)
现在，假设我们尝试从父表"customer"中删除 id = 1 或 id =2 的行(因为这两行都在子表中使用)，那么 MySQL 会由于外键约束失败而抛出如下错误。
mysql> Delete from customer where id = 1; ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`query`.`orders`, CONSTRAINT `orders_ibfk_1` FOREIGN KEY (`id`)REFERENCES `customer` (`id`)) mysql> Delete from customer where id = 2; ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`query`.`orders`, CONSTRAINT `orders_ibfk_1` FOREIGN KEY (`id`)REFERENCES `customer` (`id`))
